1. Field of the Invention
The present invention relates to an air-fuel ratio control system for an internal combustion engine of a vehicle, and more particularly relates to a system in which the air-fuel ratio of the mixture to be supplied to the vehicle engine is controlled toward a target value in response to an output signal level of an oxygen concentration sensor.
2. Description of Background Information
Air-fuel ratio feedback control systems for a vehicle internal combustion engine are known wherein the oxygen concentration in the exhaust gas of the engine is detected by an oxygen concentration sensor (referred to an O.sub.2 sensor hereinafter) and an air-fuel ratio of mixture to be supplied to the engine is feedback controlled in response to an output signal level of the O.sub.2 sensor for the purification of the exhaust gas and improvements of the fuel economy. As an example of such an air-fuel ratio feedback control system, an air-intake side secondary air supply system for the feedback control is proposed, for example, in Japanese Patent Publication No. 55-3533 in which an open/close valve is disposed in an air intake side secondary air supply passage leading to a carburetor of the engine and a duty ratio of the open and close of the open/close valve, i.e. the supply of the air intake side secondary air, is feedback controlled in response to the output signal level of the O.sub.2 sensor.
In the usual air-fuel ratio feedback control systems, it is customary to use an O.sub.2 sensor whose output signal level is not proportional to the oxygen concentration in the exhaust gas. On the other hand, an O.sub.2 sensor whose output signal level varies generally in proportion to the oxygen concentration in the exhaust gas when the air-fuel ratio of the mixture to be supplied to the engine is leaner than a stoichiometric air-fuel ratio has been developed recently. For instance, an air-fuel ratio control system using an O.sub.2 sensor of the above mentioned type for precisely controlling the air-fuel ratio toward a target air-fuel ratio in a lean air-fuel ratio is described in Japanese patant application laid open No. 58-59330.
In the air-fuel ratio feedback control system in which the air-fuel ratio is controlled by a feedback operation toward a target air-fuel ratio using such a "lean O.sub.2 sensor", the target air-fuel ratio is usually determined from a pressure within the intake pipe on the down stream side of the throttle valve, and the engine speed. However, even with this type of air-fuel ratio feedback control system, the air-fuel ratio of the mixture tends to become rich when the vehicle is running in an area of high altitude. This is because the density of the intake air of the engine becomes small in such an area, which means that the weight of the intake air is smaller than that in the normal area. Therefore, those air-fuel ratio control systems have been encountering a problem that the engine output power reduces when the vehicle is running in an area of high altitude.
Further, it is conceivable to control the air-fuel ratio of the mixture, in an area of high altitude, toward a target air-fuel ratio by means of the above described air intake side secondary air supply system. However, in that case, there is a problem of an increase of fuel consumption as a result of a delay of response of the air-fuel ratio control system. Generally, this delay of response is caused primarily by a time period required for detecting a change in the air-fuel ratio of the mixture supplied to the engine by means of the O.sub.2 sensor as a change in an oxygen concentration in the exhaust gas, and secondarily by a driving condition of the vehicle which may be varied rapidly.